Mangrove Trees: The Marine Tree Of Life?

On the border between land and sea, a variety of unique environments form. One of these environments is the mangrove forest. A mangrove forest is a habitat formed by the presence of mangrove trees on estuaries and coastal zones, and this environment usually has very fine silt that is high in organic content. This is usually a result of the high amounts of wave action that these locations receive. The mangrove trees can withstand a range of salinities (the amount of salt dissolved in water), from brackish water (water with a salinity between fresh and salt water) to concentrated sea water that is formed through evaporation. It is their ability to survive in harsh conditions that makes them so remarkable.

Why are mangroves special?

The mangrove trees (true mangrove trees are part of the genus: Rhizophora) that define this environment are found across most tropical and subtropical waters (mainly between 25° N and 25° S). The countries with the most mangrove forest cover are as follows: Indonesia, Brazil, and Malaysia. Mangroves are in between being reserved and respected, and being destroyed. This is because they are thought to provide a good buffer to the tsunamis and cyclones that the previously mentioned regions are prone to suffering from. However, they are also being damaged and destroyed as they are found at the edges of land or river estuaries where the access to the ocean is best, and therefore the land is revered as locations for towns and villages. The mangrove tree species grow exclusively in the saline environment of the intertidal zones that are termed mangrove forests. The intertidal zones where mangroves are found fluctuate in requirements frequently. At high tide, salt water is brought in, which increase the salinity of the land, water, and plants. This is only amplified one the water has evaporated, leaving concentrated salt deposits in the soil – meaning the return of the water both increases and decreases the salt content of the soil. At low tide, the plants are additionally exposed to increases in temperature, water loss, and salinity.

The edge of a mangrove forest with “root stilts”. Image: US National Oceanic and Atmospheric Administration. Wikipedia Commons.

They’re suited to this job as they are very complex trees. Their root systems have salt filtration systems and they form in intricate ways to compensate for the wave action and salt water immersion they are exposed to. These systems are in the form of an intercellular “pump” that allows them to remove excess salt from the cells within the plant. Some species also use pneumatophores (roots that grow above ground) to remain above the water they are anchored in, allowing them to respire properly. Their ability to survive and thrive in these heavily saline environments makes them halophytes (a plant that grows in high salinity environments). Mangrove trees display convergent evolution, which is the process of at least two species (that are located separately to each other) displaying similar traits – they’ve found similar solutions to the same issues that all mangrove trees face whether it be varying salinity, tidal height, oxygen lacking soils, or intense equatorial sunlight. The biodiversity of plant life found in the mangrove environment is limited, this is because the mangrove trees are both the best adapted to the environment, and other plants have not adapted to the extreme environmental requirements.

How do mangrove trees overcome the issues that they face?

In order to best overcome the issues of the environment that they live in, the mangrove tree has had to adapt. The silt that the plants root themselves on is typically very small (per grain) and has a low oxygen content. This means that in order to live, the mangrove trees have to adapt. The red mangrove tree (Rhizophora mangle) has adapted to this condition by using its ventrally (underneath) located roots as stilts to hold it further up in the water, allowing it to use its adapted bark to absorb oxygen from the air using its “lenticels”. Alternatively, the black mangrove tree (Avicennia germinans) uses the previously mentioned pneumatophores, due to growing on higher ground. These pneumatophores stick out of the soil and are also covered in lenticels. Another issue that mangrove trees face is the salinity of the water that they are settled in. Obviously, this salinity level varies depending on where the tree is in the world, but the need to exclude salt from cells is always present. This is handled in different ways depending on the plant. Red mangroves (Rhizophora mangle) exclude salts by having impermeable (meaning liquid cannot pass through it) root systems, they can additionally store excess salt in the vacuoles of their cells. Some species instead choose to excrete the salt from two salt glands at the base of each leaf. Mangrove trees also need to limit the water that they lose due to the climate that they live – consistently hot equatorial sun means water loss can be massive unless otherwise controlled, which when coupled with the limited amount of freshwater available can result in dehydration and death. They limit their water loss by restricting the size of the opening of their stomata (small holes on the underside of the leaf responsible for gas exchange during photosynthesis), as well as having the ability to change the direction their leaves face. This is used in order to avoid direct sunlight during the periods of the hottest temperatures and therefore reduce the amount of liquid lost passively.

What lives within the mangrove trees?

The position of mangrove forests at the boundary between land and sea makes an interesting biome for life. The root systems of the complex mangrove trees provide quiet shelter for young animals, especially in areas that spend all the time submerged. These areas of submersion are home to a variety of flora and fauna. The bushes and trees of the mangrove forest provide a vertical habitat for a number of mammals to survive, while the roots become home to various species of algae, molluscs, and sponges, as well as providing “nursery” zones. It is also thought that crabs found in the mangrove forests are keystone species due to their ability to process fallen mangrove leaves into nutrient rich excrement which adds to the soil and allows bottom feeders to take residence.

Research conducted by Peter Mumby in 2004 showed that mangrove forests have an effect on the species of animals that live in nearby reef systems. The research shows that populations of coral reef fish were twice as dense when the reefs were located in close proximity to the mangrove forests. Additionally, the presence of mangrove forests played a necessary part in the growth of rainbow parrotfish (Scarus guacamaia) – a species of fish that spends its life feeding on detritus, bacterial colonies, and meiofauna (small seabed dwelling invertebrates). This was found to be because the infant parrotfish would grow in seagrass, before reaching a size too big to be actively hidden, which is when they move to living within the root systems of the mangrove forests.

In the case of lemon sharks (Negaprion brevirostris), they are frequently found in and around mangrove forests. This is thought to be because of two reasons; the first being the mangrove’s use as a nursery, and the second being for hunting purposes. When lemon sharks mate, they frequently use mangrove forests as a location for spawning as they are viviparous (The pups grow inside the mother, and are born live), with the pups then use as a nursery. This is because the roots systems of the mangrove forest provide protection to the young, as well as providing a space to safely hunt which is needed if they wish to grow to maturity. This works so well because other fish species use mangroves as nurseries too, which allows the infant lemon sharks to hunt other infant fish species. As previously mentioned, they also use it as a place to hunt as juveniles/adults. The juveniles of the species will use mangrove forests as a potential place to hunt when other food sources have proven unfruitful, or when they hunt for other infant lemon sharks.

This shows that even though the mangrove forests located around the world are harsh, ever changing environments, there is still a biological interconnectivity of highly adapted plants, and symbiotic animals that live within it.

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